The invention relates to a Codec circuit and a method for increasing the data transmission rate of a modem, in which messages are transmitted digitally with pulse code modulation.
A Codec circuit is a piece of equipment which carries out PCM coding in the outgoing direction and PCM decoding in the incoming direction.
N. Warke, M. Ali xe2x80x9cOptimum Codec Companding for High-Speed PCM Data Transmission in Telephone Networksxe2x80x9d, 1999 IEEE International Conference on Acoustics, Speech, and Signal Processing, Proceedings, ICASSP99, March 1999(pages 2679 to 2682, XP002162341, Phoenix, Ariz., USA) discloses a Codec circuit having a transmission signal path and a reception signal path. During the acquisition of a specific pseudo-random number code at the start of a modem transmission, switching over to a linear compression curve is performed in order to increase the data transmission rate.
U.S. Pat. No. 4,788,692 describes an adaptive differential PCM system with a detection device for acquiring a modem signal. When a modem signal is detected, a quantizer is switched over by the detection device in order to transmit a modem signal in an optimum way.
Pulse code modulation (PCM) is a time-division multiplex system which permits multiple transmission on one line path. PCM systems are constructed and operated using digital technology. They provide a better transmission quality than comparable analog systems. PCM systems can be used for cable lines which are not suitable for carrier frequency operation. Multi-conductor cables can be used fully in the time-division multiplex method. A disadvantage of PCM systems is the increasing of the necessary bandwidth which the code-modulated signal has in comparison with other types of signals. During the transmission of signals, at the transmit end, the incoming signals are limited in the frequency band to 3.4 kHz, sampled, quantized and fed to the coder which forms the associated code words, transmitted from the transmission location to the reception location, for the successive amplitude values. At the reception location, the transmitted signals are decoded and converted into a pulse amplitude modulated signal and demodulated.
Modems are devices for transmitting data signals over telephone channels by means of modulation. In previous Codec circuits according to the prior art, the PCM coding was carried out in the outgoing direction and PCM decoding was carried out in the incoming direction, regardless of whether the signals to be coded originate usual voice signals from a telephone apparatus or from a modem. The transmission properties of known Codec circuits are the same for the coding of telephone signals and modem signals. The result of this is that the data transmission of modem signals takes place in the same frequency bandwidth as the data transmission of telephone voice signals, namely within a frequency band which extends from approximately 100 Hz as the lower limiting frequency up to approximately 3.4 kHz as the upper limiting frequency. Because the data transmission rate is directly proportional to the frequency bandwidth of the frequency band which can be used, the transmission rate in Codec circuits is limited by the transmission frequency band which extends from approximately 100 Hz to 3.4 kHz.
The object of the present invention is therefore to provide a method and a Codec circuit for increasing the data transmission rate in a modem data transmission.
In an advantageous embodiment of the Codec circuit according to the invention, the programmable digital filters are bandpass filters with a lower limiting frequency and an upper limiting frequency.
The setting coefficients of the digital switchover filters can preferably be stored in the associated coefficient memory.
This provides the particular advantage that the transmission properties can be adapted rapidly and flexibly to the transmission requirements of the modem circuit by storing other setting coefficients of the digital switchover filters.
The coefficient memories of the Codec circuit according to the invention are preferably connected to a coefficient input device via setting lines.
In one preferred embodiment of the Codec circuit according to the invention, the digital switchover filters are connected at the output end to a summing device.
The digital switchover filters of the Codec switches according to the invention are preferably filters of the seventh order.
In one preferred embodiment of the Codec circuit according to the invention, the modem signal detection device detects a modem signal by acquiring an initial signal tone with a predetermined signal frequency at the start of the modem transmission.
The transmission and reception signals which are transmitted on the transmission signal path and the reception signal path are preferably PCM signals.
A preferred embodiment of the Codec circuit according to the invention will be described below in order to explain features which are essential to the invention, with reference to the appended drawings.